1. The Field of the Invention
This invention relates to vehicle equipment, and, more particularly, to auxiliary trailing axles for trucks.
2. The Background Art
Highway construction and maintenance is a matter of substantial concern to local, state, and federal governments. Road construction has always been an expensive proposition. Roads constructed using modern knowledge, methods, and technology have greatly improved the load-bearing capacity of vehicles traveling over those roads.
Specific limitations exist on loading of axles. It is well established that bridges are designed to carry specific weights. However, in actual bridge design, several additional, localized factors exist. For example, a bridge may have a surfacing material such as concrete or asphalt. These may be designed in various compositions to support various loads and provide predictable durability. However, underlying a bridge or road surface is a structure of specific members each designed for supporting a particular maximum force or load.
Bridges in various parts of a roadway system have varying weight-carrying capacities. A truck having weight over some number of axles, must also have those axles distributed across a suitable length of the bridge in order to distribute the load of the truck properly over individual structural members of the bridge.
Thinking in terms of a truck, not as a truck, but as a series of axles, each bearing a load, one sees another important factor in the mutual design criteria between vehicles and roadways (e.g. bridges). That is, axles cannot be separated from the truck. The truck has a length; therefore, axles cannot be completely separated from each other. Therefore, all of the axles of the truck will pass over the bridge together. The truck has to distribute axles with some maximum length.
Moreover, road construction does leave all streets, highways, roads, and the like with specific limitations on sustainable loads and the like. For example, just as building construction must start far below the surface level of the earth in order to support a foundation, a road bed is deeply laid for many roads. Above a road bed are laid various types and grades of materials. Ultimately, a surface material is provided on which vehicles roll directly.
A fundamental engineering principal is involved in the concept of maximum stress and principal stress directions. In solid materials of uniform, isotropic, properties, principal stresses are compression tension, and shear. At any location, principal stresses may be axial or may be resolved axially. Accordingly, once stresses have been resolved along orthogonal axes, all loads may be represented as either tension or compression. However, in a material subjected to any combination of tension, compression, or both, along principal axes, shear stress is induced at an angle with respect to the principal stresses.
Therefore, many years of stress analysis have developed by reliance on a host of methods. Nevertheless, in terms of understanding the principal stress planes, Mohr's circle is an engineering construct useful for explaining the directions and magnitudes of principal stresses.
In accordance with these concepts of principal stresses, as known in the art, certain design approaches may be used to minimize stress or support stress as needed. One important principal is St. Venant's principal. St. Venant's principal may be thought of in terms of principal stress. Accordingly, whenever stress is localized, by a load on a solid structure, the load will be distributed on an angle corresponding to the angle of the principal shear planes.
Therefore, one may think of a road bed as a pile of rocks of varying sizes and qualities. Nevertheless, each individual particle in a road bed or a road surface material experiences stress from a load according to its comparative distance from the point of application of the load. Therefore, as a practical matter, treating the load as a major force component on the road, and assuming isotropic properties, one may imagine loads being distributed at an angle of 45 degrees away from the direct or normal load applied to a road surface. Accordingly, two feet below a road surface, a load may be distributed two feet away from the point of application of the load in each horizontal direction, assuming that the load is normal to the surface. In reality, road materials are not solid. Thus, each particular solid particle is, or may be, heavily, locally loaded.
Also, competing considerations exist in road construction. For example, a layer of aggregate does not support tension. It only supports compression. However, rocks neighboring a loaded rock can restrain the loaded rock from moving. Thus, the concept of principal stress is a useful concept in understanding the damage that may be done to a road.
One reason why trucks, cars, and vehicles in general rely on pneumatic tires is to improve the ability of the vehicle to absorb shocks from the roughness of a surface. Nevertheless, another purpose of rubber tires is to distribute the load of the vehicle over a surface area of a road surfacing material. Tire pressures relate directly to the distortion of a tire in order to present a certain amount of area onto a road for supporting the weight of the vehicle.
For example, a four thousand pound vehicle having a total of fifty square inches of tire surface to the road must have a tire pressure of approximately twenty pounds per square inch in order to support the load. To support the same load or weight of a vehicle at forty pounds per square inch only twenty-five square inches of tire tread must be in contact with the road. Thus, local pressure on a road surface may be controlled, to a certain extent, by the inherit limits on tire pressures.
However, further down through a road bed away from a tire running on a road surface, the total force of a tire has been integrated by St. Venant's principal. One may note that two axles, close together will produce more load in a road bed than the same two axles, carrying the same two loads, but spaced a substantial difference apart, with respect to the thickness of the road bed.
Thus, one may see that axle location may be very important, as is the net, local force presented on a bridge or a road bed by an axle. In this context, an axle may be used to refer to the axle itself, or to the axle and tires as they represent force application to a road bed from a vehicle supported thereby.
Trucks today may be manufactured to have tandem axles spaced a comparatively long distance apart, as compared with trucks of previous years. Also, trucks now carry auxiliary axles that can be engaged for distributing a load along a different length of the truck. For example, long truck bodies or trailers may have wheels located nearer the front end, rather than leaving the entire weight distributed between a front axle and a rear axle or between a tractor and a pair of closely spaced tandem axles at the rear.
Auxiliary axles are added to concrete mixer trucks to accommodate limitations on bridge weights. Also, auxiliary axles may be added to accommodate the large differential load between an empty truck and a loaded truck. Thus, auxiliary axles may be engaged for a limited time, only while a vehicle is loaded and is traveling on a road. At a work site, a truck may not need auxiliary axles as a support for the vehicle itself, and may disengage them.
Thus, heavily loaded trucks having changes in load actually applied thereto, may need auxiliary axles. Those axles need to be distributed along a maximum length, and may need to be distributed along the vehicle itself To protect roadways, to satisfy bridge weight limitations, and to support substantial loads, auxiliary axles may be used in vehicle construction.
Auxiliary axles themselves present various problems. One may think of the problems as difficulties that auxiliary axles do not solve. In some instances auxiliary axles introduce new problems of their own to vehicle construction, legal compliance of vehicles with road and bridge limits, or with operation of a vehicle in transit or on a job for which the vehicle is designed.
For example, trailing axles have been added to concrete mixer trucks and dump trucks. Auxiliary axles sometimes drop down from in front of our behind the main, tandem or single, rear, load-bearing axles. In some applications, trucks may be fitted with trailing axles that extend behind the main body of a truck. Brackets may be fitted to a frame in order to carry arms or booms that hydraulically actuate in order to lift or drop an axle for service.
In some instances, some concrete mixer trucks draw a separate frame, hinged as a trailer, and yet being permanently attached to the main frame of the truck. Thus, the net length of the rigid frame is shortened, while the engine, even if it drives the vehicle running gear itself, is off-loaded away from the main load.
Some trucks require a short turning radius, due to their operation. For example, refuse trucks may use add-on or auxiliary axles in various configurations. However, they typically are positioned directly in front of or behind the main load-bearing axles at the rear of the vehicle. Even trailing axles as disclosed in U.S. patent application Ser. No. 08/893,600, incorporated herein by reference, introduced a host of difficulties that are not presently tractable for certain trucks.
For example, refuse truck bodies contain large forces due to the compression mechanisms that compress the refuse stored therein. Tailgates and bodies of refuse trucks often have a circular configuration when viewed longitudinally in cross-section. Moreover, and more importantly, when viewed laterally or transversely (orthogonally to a longitudinal direction), a refuse truck tailgate may actually have a bubble. In cross-section, a bubble may be a portion of a circle, whether or not it is a sphere (semi-sphere). The resulting curvature reduces bending stress at the joint (e.g. between the tailgate, end cap, or "bubble", and the frame holding the bubble). The frame effects fastening of the tailgate containing the bubble to the main body or containment vessel of the truck.
Pressure vessels like the tailgate are round for a reason. The material is generally loaded in tension reducing the need for additional stiffeners. Accordingly, reduced stress may be provided by a semi-cylindrical, semi-spherical shape (a "bubble") in order to accommodate loads at minimum stress.
However, the bubble typically extends beyond the frame of the vehicle. Extending beyond a frame of a vehicle presents several problems. One problem is that a structure placed behind the vehicle as a load-bearing auxiliary axle and mounting assembly must accommodate the envelope of the vehicle body. On the one hand, length may be good for distributing load away from the other load-bearing axles of a vehicle. On the other hand, length cannot exceed length limits that exist for various licensing and regulatory limitations of vehicles.
Likewise, trucks are manufactured in specific shapes for performing certain functions. To the extent that an auxiliary axle interferes with the operation of truck, the auxiliary axle must be removed as an obstruction to operation, while being deployable for road transit. In certain embodiments, an auxiliary axle may be lifted or pivoted to remove wheels or supporting frame structures away from a tailgate of a vehicle, such as a truck. In such embodiments, the very length that may provide a distribution wheelbase for a load becomes a clearance problem in altitude, width, or other operational clearance.
The hydraulic mechanisms for deploying and supporting auxiliary axles may be problematic. Hydraulic actuation is not designed for supporting the loads and the frequency of cycling loads that can be expected in a suspension system of a truck. Hydraulic loading mechanisms make very poor structures for primary suspension systems on auxiliary axles.
What is needed is a new system of auxiliary axles for installation on heavy trucks, such as dump trucks, concrete mixer trucks, and especially for refuse trucks. Refuse trucks specifically need a very short coupling for the envelope of the auxiliary axle when stored (stowed) in a non-load-bearing configuration. By the same token, a refuse truck needs castering wheels extended a maximum distance, within the length limits permissible, away from the tailgate of a truck.
Likewise, a refuse truck needs to use an auxiliary axle that will accommodate a bubble on a tailgate. The tailgate of a refuse truck often opens directly to discharge the load. In certain embodiments, the entire tailgate structure may lift or swing on a pivot to completely clear the entire interior cross-sectional area of the pressure vessel that forms the refuse-holding body. Accordingly, an auxiliary axle must clear all structures away from the space behind the body and below the tailgate. To do otherwise is to present substantial difficulties to structure, clutter, operational simplicity, health, and so forth.
Dump trucks tend to be shorter than refuse trucks. Auxiliary axles, particularly trailing axles that may be well adapted to dump trucks, are sometimes not well suited to refuse trucks. A truck having a single frame, rather than having a semi-tractor-trailer rig or an articulated, trailing portion, has an overall length limit in many states of about forty to forty-five feet. Auxiliary axle systems such as a trailing axle suitable for dump trucks, if attached to the rear of this type of refuse truck would extend longer than the required forty foot limit. In order to allow enough clearance in both altitude and width to fit around a bubble on a tailgate of a refuse truck, a conventional axle assembly would extend too high, typically, or extend too long in service.
What is needed is a method and apparatus for coupling a trailing, auxiliary, load-bearing axle assembly that may be designed to fit the envelope of maximum running length, maximum load-distribution length, maximum axle-separation distances, maximum top clearance, and so forth, while meeting the operational needs, transit requirements, structural limitations, suspension response criteria, and interstate commerce commission (ICC) safety considerations. What is needed is a trailing, auxiliary axle (system, assembly, etc.) that can be selectively engaged and disengaged for transit, and may be selectively removed or stowed or otherwise placed out of the way during functional operations at a source site or destination site where operations occur, such a systems needs to meet the suspension requirements in terms of loading, response times and distances, vehicle support and shock absorption from terrain and road variations expected in service (transit or operations). It should provide the maneuverability required for passage through depressions and ditches, over bumps, humps, curbs, construction banks, on roads or off roads. It should turn within residential cul-de-sacs, turn around on a roadway, back in and out of confined spaces and so forth. What is needed in particular is such functional features for refuse trucks.
In refuse trucks, legal weight limitations according to the bridge regulatory requirements on loading of axles is a substantial limitation. Moreover, a refuse truck requires a substantial amount of its time for turn-around travel. That is, a refuse truck will travel on pickup rounds during a portion of operations. Thereafter, the truck must return to a dumping, collection, or disposal area. The truck must then return to the pickup area where work was interrupted for traveling to discharge the load.
Thus, much time could be saved if, for example, a refuse truck could increase it's load by 30, 50, or even 100 percent. A refuse truck could then either operate longer in an individual area, or cover a comparatively more extensive route looping away from and back toward the discharge site. It may simply require fewer return trips to a distant site. If the load-bearing capacity of refuse trucks could be improved while fitting within the regulatory requirements for road and bridge loading by axles, an extremely significant increase in the efficiency, man-power, traffic, cycle times, and equipment utilization, may be achieved.
One may think of the tare weight of a loaded refuse truck as the actual, empty vehicle weight. The vehicle weight may be comparable to that of the load. Therefore, doubling the carrying capacity (load) does not require doubling the permissible gross (licensed, regulated) weight or the structural capacity of the truck. Realistically, bodies of refuse trucks can be designed, and many are currently designed, to support the increased pressures of compaction, and structural requirements of supporting greater loads than can be legally carried on their current axle distribution systems under present regulations.
Finally, current technology in trailing, auxiliary axle systems relies extensively on the hydraulic designs used for actuation of the framing structures (booms, beams, etc.) that make up the trailing axle assembly. Since conventional wisdom dictates hydraulic loading downward toward a boom, in order to lift a truck against the support being provided by the trailing axle from the road, the hydraulic systems must absorb road shocks. The substantial weight, of both hydraulic oil and structural masses of cylinders, pistons, frames, and the like, can dramatically change the response of a trailing axle to road impacts.
Even with shock absorption mechanisms such as accumulators, gas springs in the hydraulic reservoirs, and the like, suspension response for trailing auxiliary axles is often not consistent with that for the production axles of a truck so equipped. The geometries of many conventional trailing auxiliary axles, have often not provided operational space for conventional suspension systems (e.g. operationally equivalent to those factory-installed on the truck itself). Thus, some trailing auxiliary axles are suspended, or suspend the vehicle, inappropriately, typically more stiffly, and thus transmit structural shocks directly to the vehicle. Moreover, most auxiliary axle schemes place the wheels in an awkward position, such as right at the bubble zenith.
Partly due to such difficulties, conventional trailing auxiliary axles are tightly coupled to retractable frame assemblies that are themselves fastened securely to the underlying frame of the truck. What is needed is a suspension system more thoroughly adapted to proper suspension systems having the same frequency of response, load-bearing capacity, load distribution, load sharing distribution with respect to other axles on the vehicle, as well as the other restrictions on envelope, clearance height, ICC bumper or ICC bar height, axle separation distances, maximum weight, and so forth.